2 3 Basic Types of Rocks Igneous Crystalline Cooling and hardening of magmaMake up the majority of crustE.g., pumiceSedimentaryresult of the accumulation of small pieces broken off of pre-existing rocksClastic: little pieces of broken up rock which have piled up and compactedChemical: minerals left behind from water evaporationOrganic: debris caused by organic processesE.g., sandstoneMetamorphicRock that has undergone chemical or structural changes due to heat and pressureE.g., marble
3 Some Vocab Subsidence - downward shift of a surface Uplift - upward shift of a surfacePlate tectonics - theory which describes the large scale motions of Earth's lithosphere
4 Uniformitarianism versus Catastrophism James Ussher (1581–1656)Earth's history has been dominated by cataclysmic events rather than gradual processes acting over long periods of time.E.g., formation of Rocky Mountains due to a single rapid event such as a great earthquakeUniformitarianism ("The present is the key to the past.”)James Hutton (1726–1797)Geologic processes operate at the same rates and with the same intensity now as they did in the past.Weathering of rocks and erosion of sedimentProblem: Impact craters and fossil record
5 The Theory of Actualism Laws of nature do not change with timeEarth's history can be explained in terms of currently observable processes.But the rates of geologic change are not constant over long periods of timeThere have been some catastrophic geologic events
6 Finding Clues about Earth’s Past FossilsRelative DatingUnconformitiesRock Layer CorrelationRadiometric Dating
8 Formation of Fossils Original remains Entire organism is preserved in its entirelyE.g., frozen organisms, insects trapped in sapReplaced remainsSoft part of organism is replaced with mineralsE.g., Fossil bones, teeth and shellsMolds and castsHollow depression is formed in the rock - MoldMinerals seep into the mold - CastE.g., shell fishTrace FossilsImpressionsE.g., footprints, burrowsCarbonaceous filmSilhouette composed of a carbon filmHigh T and P cause organic compounds to undergo chemical change
9 Organism must die in or near water Remains are insulated from elements that contribute to decompositionOnly soft part decomposesSedimentationRemains are buried (soil, mud and land slides)Rapid sedimentation --> great for fossilizationClay versus sandPermineralizationLower sediment layers become compactedPressure on fossil increases, turning sediments to rockMinerals glue particles of sediment togetherMinerals may replace hard remainsUpliftMovement of platesFormation of mountainsSea floors become dry landErosionReveals fossils
10 Estimating Time Periods Using Relative Dating Relative Dating - placement of events in sequence (not actual dates)Stratigraphic LawsEmbedded Fragments
11 Stratigraphic Laws Law of Superposition Law of Original Horizontality Oldest layer is at the bottom of the sequenceLaw of Original HorizontalitySediments and rock layers were deposited horizontallyTopography controls the angle at which sediments are deposited locally.
12 Stratigraphic Laws Law of Lateral Continuity Deposits originally extended in all directionsCross-Cutting RelationshipRock which intrudes by magma flow into existing rock is always younger than the rock it invades.Igneous intrusion
13 Embedded FragmentsRocks embedded in another rock are older than the rock in which they are foundPebbles in a conglomerate must have existed before the conglomerate formed.
14 UnconformitiesGaps in the geologic record that may indicate episodes of deformation, erosion, and sea level variations.times when deposition stopped, an interval of erosion removed some of the previously deposited rock, and finally deposition was resumed.
15 Angular UnconformityOlder package of sediments has been tilted, eroded, and then erosion, younger package of sediments was deposited on this erosion surface.Sediment depositionrocks are uplifted and tilted (deformation)erosion removes the uplifted mountain range;sea covers the land surfacenew sediments are deposited
16 Disconformities Erosion surface between two packages of sediment lower package of sediments was not tilted prior to deposition of the upper sediment package.Subsidence and sediment deposition;uplift and erosion;Third: renewed subsidence and deposition.
17 Correlating Rock Layers Matching layers from two locationsWalking the OutcropOutcrop - part of a layer that can be seen at the surfaceEasy way fo find if two layers are the sameDifficulty if there is vegitation, erosion or thick soilMatching Rock CharacteristicsMatching Key BedsSingle rock layer that is unique, easily recognizable and widespreadBentonite (clay material formed from volcanic eruptions)Dust and debris from impact cratersIndex FossilsUniqueAbundantFound over a broad geographical areaOccur only in a few rock layers (same time period formation)
18 NonconformitySedimentary layers are deposited on igneous or metamorphic rockIndication of long periods of erosion prior to depositionRecord of major episodes of uplift, erosion and subsidence during growth of the continentsEvidence for mobility of the crust
19 Measuring Absolute Time Rates of Erosion and SedimentationNot constantOnly the ages of young geologic featuresCounting Tree rings1 ring = 1 yearWidth = rainfall and temperatureVarveSediment that is deposited on a yearly cycleClearest in glacial lakes formed during an ice ageThick, light colored sandy layer in summerThin, dark-colored clay layer in the winterRadiometric Dating
20 Atomic Structure So, mass number (A) = #p+ + # n0 Subatomic ParticleLocationChargeMass (g)Mass (u)ProtonNucleusPositive1.67 x 10-241NeutronNeutralElectronOrbiting the nucleusNegative9.28 x 10-28= 0u = atomic mass unit (1 u = 1.67 x g)So, mass number (A) = #p+ + # n0The mass number of an element is represented as relative mass numberssince the atoms are so small. The scale used is that of C-12 (C-12 = 12u).Standard Atomic Notation: AXZ
21 Isotopes Notice the masses in the periodic table! They are not integers!The masses found in the periodic table are actually called atomic masses. They are weighted average of the mass numbers of the atoms of an element.Atoms of an element have different number of neutrons. They have the same number of protons, because Z is unique to the elementIsotopes are forms of an element in which the atoms have the same number of protons but different number of neutrons.
22 Isotopes Atomic mass = 75.77% x 35 + 24.23% x 37 = 35.45 Symbol ProtonsNeutronsElectronsAbundance35Cl17Chlorine-351875.77%37ClChlorine-372024.23%Atomic mass = 75.77% x % x 37 = 35.45Protons and electrons are largely responsible for the chemical behaviourof an element, therefore isotopes may have the same chemical properties.But physical properties may vary. E.g., heavy water (H-2 aka deuterium)Is used in nuclear reactors.
23 RadioisotopesThe isotopes of some elements are very unstable; they emit radiation when they decay, changing the composition of its nucleusThese isotopes are called radioisotopesThe radiation emitted can be either harmless or very dangerous
24 Types of Radiation Radiation Approximate Speed Penetration in air Effective BarrierAlpha (a, 4He2+)2Variable, but relatively slowA few cmA sheet of paperBeta (b, e-)Variable, but relatively fastA few m1-2 mm of metalGamma (g)Very fast (speed of light)Unlimited1 m of lead or concrete
25 Radioisotopes and Half-Life Radioisotopes have characteristic half-lives.A half-life (T1/2) is the time it takes half of the nuclei in a radioactive sample to decay.Radiocarbon Dating (C-14 --> C-12)Great for dating fossils (C-14… T1/2 = 5730 years!). Ratio of C-14 to C-12: time elapsed since the death of the organismUranium-Lead Dating (U > Pb-206)T1/2 for uranium is 4.5 billion yearsDating of oldest rocksRubidium-Strontium Dating (Rb-87 -->Sr-87)T1/2 is 47 billion yearsDating of extremely old rocksPotassium-Argon Dating (K-40 -->Ar-40)T1/2 is 1.3 billion yearsK is very common in certain mineral rocks